The supervisors were believed to have opened fire on Wednesday (local time) at a crowd of about 200 mostly Bangladeshi immigrants who were demanding wages that had not been paid, police said. The wounded were taken to hospital but none of the injuries was serious.
Anti-foreigner sentiment has been rising in Greece, where one in four workers is unemployed after five years of recession.
Police said they had arrested the owner of the farm, in the southwestern town of Manolada, and were still hunting the foremen.
One of the immigrants involved in the protests told Greek Skai TV that they had been promised wages of 22 euros ($28.70) a day.
"They keep telling us that we will get paid in a month, and this has been going on for more than a year," said the worker, who was not identified. "We don't talk about it because we are afraid that we will be killed or kicked out."
Greece is a gateway for mostly Asian and African migrants trying to enter the European Union through its porous sea and land borders.
Most of those who find work in Greece are employed illegally; more than 40 percent of Greece's informal workers are migrants.
The Council of Europe's Commissioner for Human Rights, Nils Muiznieks, said after visiting Greece this year that he was seriously concerned about a rise in racist violence and urged authorities to get tougher.
Government spokesman Simos Kedikoglou on Thursday condemned what he called an "inhuman attack".
"This unprecedented and shameful act is foreign to Greek ethics," he said.
